Accidental fuse programming protection circuits

Abstract

Apparatus and methods for protection against inadvertent programming of fuse cells are provided herein. In certain configurations, a fuse system includes a fuse programming transistor, a cascode transistor, and a fuse cell electrically connected in series between a first pad and a second pad. The fuse system further includes a bias generator that controls an amount of current provided to the fuse cell based on biasing a gate of the fuse programming transistor and a gate of the cascode transistor. The fuse system further includes a fuse protection capacitor electrically connected between the first pad and the gate of the cascode transistor to prevent inadvertent programming of the fuse cell in response to an increase in voltage of the first pad relative to the second pad.

What is claimed is: 
     
       1. A fuse system for a semiconductor die, the fuse system comprising:
 a plurality of pads including a first pad and a second pad; 
 a plurality of switches including a first switch and a second switch; 
 a fuse electrically connected in series with the first switch and the second switch between the first pad and the second pad; 
 a biasing circuit configured to bias a control input of the first switch with a fuse programming signal to thereby control a current through the fuse; and 
 a fuse protection capacitor electrically connected between the first pad and a control input of the second switch, the fuse protection capacitor operable to inhibit unintended programming of the fuse by adjusting a bias of the second switch in response to a change in a voltage of the first pad relative to a voltage of the second pad. 
 
     
     
       2. The fuse system of  claim 1  wherein the biasing circuit includes a voltage regulator electrically connected between the first pad and the second pad and configured to generate a regulated voltage, and a programming logic circuit configured to generate the fuse programming signal based on the regulated voltage. 
     
     
       3. The fuse system of  claim 2  wherein the voltage regulator is further configured to bias the control input of the second switch, the fuse protection capacitor operable to prevent accidental programming of the fuse arising from a delay of the voltage regulator in providing voltage regulation. 
     
     
       4. The fuse system of  claim 3  wherein the voltage regulator is a low dropout regulator. 
     
     
       5. The fuse system of  claim 2  wherein the programming logic circuit includes a level shifter configured to generate the fuse programming signal by level shifting a control signal, the level shifter configured to control the fuse programming signal to a first voltage level about equal to the voltage of the first pad in a first state of the control signal, and to control the fuse programming signal to a second voltage level about equal to the regulated voltage in a second state of the control signal. 
     
     
       6. The fuse system of  claim 2  further comprising a serial interface configured to provide a control signal to the programming logic circuit to instruct programming of the fuse. 
     
     
       7. The fuse system of  claim 1  wherein the first switch is implemented as a fuse programming field-effect transistor, and the second switch is implemented as a cascode field-effect transistor. 
     
     
       8. The fuse system of  claim 1  wherein the first pad is a shared power supply and fuse programming pad and the second pad is a ground pad. 
     
     
       9. The fuse system of  claim 1  further comprising a fuse sensing circuit electrically connected to the fuse and configured to detect a state of the fuse. 
     
     
       10. The fuse system of  claim 1  wherein the biasing circuit is further configured to bias the control input to the second switch based on a voltage difference between the first pad and the second pad. 
     
     
       11. A method of protecting from accidental fuse programming, the method comprising:
 biasing a control input of a first switch with a fuse programming signal; 
 controlling a current through the fuse based on the fuse programming signal, the fuse electrically connected in series with the first switch and a second switch between a first pad and a second pad; and 
 inhibiting unintended programming of the fuse by adjusting a bias of the second switch using a fuse protection capacitor in response to a change in a voltage of the first pad relative to a voltage of the second pad, the fuse protection capacitor electrically connected between the first pad and a control input of the second switch. 
 
     
     
       12. The method of  claim 11  further comprising biasing the control input of the second switch based on a voltage difference between the first pad and the second pad. 
     
     
       13. The method of  claim 11  further comprising generating a regulated voltage using a voltage regulator that is electrically connected between the first pad and the second pad, and generating the fuse programming signal based on the regulated voltage. 
     
     
       14. The method of  claim 13  further comprising biasing the control input of the second switch using the voltage regulator, and preventing accidental programming of the fuse arising from a delay of the voltage regulator in providing voltage regulation. 
     
     
       15. A packaged module comprising:
 a package substrate; and 
 a semiconductor die attached to the package substrate, the semiconductor die including a first switch, a second switch, and a fuse electrically connected in series with one another between a first pad and a second pad, the semiconductor die further including a biasing circuit configured to bias a control input of the first switch with a fuse programming signal to thereby control a current through the fuse, and a fuse protection capacitor electrically connected between the first pad and a control input of the second switch, the fuse protection capacitor operable to inhibit unintended programming of the fuse by adjusting a bias of the second switch in response to a change in a voltage of the first pad relative to a voltage of the second pad. 
 
     
     
       16. The packaged module of  claim 15  wherein the biasing circuit includes a voltage regulator electrically connected between the first pad and the second pad and configured to generate a regulated voltage, and a programming logic circuit configured to generate the fuse programming signal based on the regulated voltage. 
     
     
       17. The packaged module of  claim 16  wherein the voltage regulator is further configured to bias the control input of the second switch, the fuse protection capacitor operable to prevent accidental programming of the fuse arising from a delay of the voltage regulator in providing voltage regulation. 
     
     
       18. The packaged module of  claim 16  wherein the programming logic circuit includes a level shifter configured to generate the fuse programming signal by level shifting a control signal, the level shifter configured to control the fuse programming signal to a first voltage level about equal to the voltage of the first pad in a first state of the control signal, and to control the fuse programming signal to a second voltage level about equal to the regulated voltage in a second state of the control signal. 
     
     
       19. The packaged module of  claim 15  wherein the first pad is a shared power supply and fuse programming pad and the second pad is a ground pad, the semiconductor die further including a core circuit configured to receive power from the first pad and the second pad. 
     
     
       20. The packaged module of  claim 15  wherein the biasing circuit is further configured to bias the control input to the second switch based on a voltage difference between the first pad and the second pad.